An optical fiber (i.e., a glass fiber typically surrounded by one or more coating layers) conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the optical cladding ng (i.e., nc>ng).
For short-distance applications and for local networks, multimode optical fibers are frequently used. The core of a multimode optical fiber typically has a diameter of between about 50 microns and 62.5 microns, whereas the core of a single-mode optical fiber typically has a diameter of between about 6 microns and 9 microns. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber, conveying the same information. The bandwidth is directly linked to the group velocity of the optical modes propagating in the multimode core of the optical fiber. Thus, to guarantee a broad bandwidth, it is desirable for the group velocities of all the modes at a given wavelength to be identical. Stated differently, the intermodal dispersion should be zero, or at least minimized, for a given wavelength.
Multimode optical fibers having a diameter of 50 microns have been the subject of international standardization under the ITU-T G.651.1 recommendations, which, in particular, define criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility. The ITU-T G.651.1 standard (July 2007) is hereby incorporated by reference in its entirety.
For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “triangular” profile, or “parabolic” profile (e.g., a graded profile or an “alpha” profile) for graphs having the respective shapes of a step, a trapezoid, a triangle, or a parabola. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
For the same propagation medium (i.e., in a step-index multimode optical fiber), the different modes have different group delay times. This difference in group delay times results in a time lag between the pulses propagating along different radial offsets of the optical fiber. This delay causes a broadening of the resulting light pulse. Broadening of the light pulse increases the risk of the pulse being superimposed onto a trailing pulse and reduces the bandwidth (i.e., data rate) supported by the optical fiber.
To reduce intermodal dispersion, the multimode optical fibers used in telecommunications generally have a core with a refractive index that decreases progressively from the center of the optical fiber to its interface with a cladding (i.e., an “alpha” core profile). Such an optical fiber has been used for a number of years, and its characteristics have been described in “Multimode Theory of Graded-Core Fibers” by D. Gloge et al., Bell system Technical Journal 1973, pp. 1563-1578, and summarized in “Comprehensive Theory of Dispersion in Graded-Index Optical Fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of the above-referenced articles is hereby incorporated by reference in its entirety.
A graded-index profile (i.e., an alpha-index profile) can be described by a relationship between the refractive index value n and the distance r from the center of the optical fiber according to the following equation:
                    n        =                              n            0                    ⁢                                    1              -                              2                ⁢                                                                  ⁢                                                      Δ                    ⁡                                          (                                              r                        a                                            )                                                        α                                                                                        (                  Equation          ⁢                                          ⁢          1                )            
wherein,
α≧1, and α is a non-dimensional parameter that is indicative of the shape of the index profile;
n0 is the maximum refractive index of the optical fiber's core;
a is the radius of the optical fiber's core; and
                    Δ        =                              (                                          n                0                2                            -                              n                cl                2                                      )                                2            ⁢                          n              0              2                                                          (                  Equation          ⁢                                          ⁢          2                )            
where nc1 is the minimum refractive index of the multimode core, which may correspond to the refractive index of the outer cladding (most often made of silica).
A multimode optical fiber with a graded index (i.e., an alpha profile) therefore has a core profile with a rotational symmetry such that along any radial direction of the optical fiber the value of the refractive index decreases continuously from the center of the optical fiber's core to its periphery. When a multimode light signal propagates in such a graded-index core, the different optical modes experience differing propagation mediums (i.e., because of the varying refractive indices). This, in turn, affects the propagation speed of each optical mode differently. Thus, by adjusting the value of the parameter α, it is possible to obtain a group delay time that is virtually equal for all of the modes. Stated differently, the refractive index profile can be modified to reduce or even eliminate intermodal dispersion.
Typically, a multimode optical fiber should have the largest possible bandwidth to perform well in a high-bandwidth application. The TIA-492AAAC-A standard, which is hereby incorporated by reference in its entirety, specifies the performance requirements for 50-micron-diameter multimode optical fibers used over long distances in Ethernet high-bandwidth transmission network applications. In particular, the OM3 and OM4 standards, each of which is hereby incorporated by reference in its entirety, have been adopted to meet the demands of high-bandwidth applications (i.e., a data rate higher than 1 GbE) over long distances (i.e., distances greater than 300 meters). The OM3 standard requires, at a wavelength of 850 nanometers, an effective modal bandwidth (EMB) of at least 2,000 MHz·km. The OM3 standard assures error-free transmissions for a data rate of 10 Gb/s (10 GbE) up to a distance of 300 meters. The OM4 standard requires, at a wavelength of 850 nanometers, an EMB of at least 4,700 MHz·km to obtain error-free transmissions for a data rate of 10 Gb/s (10 GbE) up to a distance of 550 meters.
For a given wavelength, the bandwidth of an optical fiber may be characterized in several different ways. In particular, the characterization of bandwidth depends on the source used with the optical fiber. On the one hand, determination of the overfilled launch bandwidth OFL-BW assumes the use of a light source exhibiting uniform excitation over the entire radial surface of the optical fiber (e.g., using a laser diode or light emitting diode (LED)). On the other hand, the effective modal bandwidth EMB is more appropriate for determining the optical fiber bandwidth in use with VCSEL sources (Vertical Cavity Surface Emitting Laser).
The effective modal bandwidth can be calculated by measuring the delay caused by modal dispersion (i.e., the dispersion mode delay, or DMD). An exemplary method of measuring DMD and calculating the effective modal bandwidth can be found in the FOTP-220 standard and the IEC 60793-1-49 standard, each of which is hereby incorporated by reference in its entirety. To carry out a DMD measurement, care is generally taken to use a source or an optical fiber length such that the chromatic dispersion is actually negligible, because the purpose is to characterize the modal dispersion of the fiber. The effective modal bandwidth corresponds to the smallest bandwidth for all the EMBs of Source-Optical fiber pairs when the chromatic dispersion is disregarded for all standardized sources in 10 GbE applications.
A DMD graph is obtained by successively injecting into the multimode optical fiber a light pulse having a given wavelength λ0 with a radial offset between each successive pulse. The delay of each pulse is then measured after a given length of fiber L. Multiple identical light pulses (i.e., light pulses having the same amplitude, wavelength, and frequency) are injected with different radial offsets with respect to the center of the multimode optical fiber's core. From these measurements a map 23 of the DMD, or “DMD graph,” may be generated depicting the pulse delay (e.g., in nanoseconds) as a function of radial offset (e.g., in microns).
When the parameter a is set to an optimum value αoptimum, there is virtually no shift in the light pulse delay for a given wavelength λ0 regardless of the radius r along which the pulse propagates. Thus, the intermodal dispersion is low and the effective modal bandwidth is significant. Commonly-assigned European Publication No. EP 2144096, which is hereby incorporated by reference in its entirety, describes an exemplary DMD graphical representation and calculated EMB.
U.S. Patent Publication No. 2010/0154478, which is hereby incorporated by reference in its entirety, discloses graded-index profiles in which the core follows a modified power law equation. The inner part follows the standard Equation 1, while the refractive index profile deviates from the power law equation with smaller refractive index. Such refractive index profiles lead to DMD degradations and limited effective modal bandwidth.
U.S. Pat. No. 7,315,677, which is hereby incorporated by reference in its entirety, discloses “double alpha profiles” based on co-doping. Each dopant profile exhibits its own alpha. DMD performances are not disclosed. Based on calculations, the disclosed optical fibers have relatively small numerical apertures of about 0.2 or less.
International Publication No. WO 00/50936, which is hereby incorporated by reference in its entirety, discloses graded-index profiles in which the core's refractive index profile follows a modified power law equation providing a DMD plot at 1300 nanometers with two different slopes over two different regions of the core. This publication, however, fails to disclose the exponent controlling its power law equation or a DMD plot at 850 nanometers.
Japanese Publication No. 2007-272239, which is hereby incorporated by reference in its entirety, is related to the compensation of the deleterious effect of the tail/skirt at the core cladding interface inherent to VAD process. The publication, however, fails to disclose a central core maximum refractive index and is, therefore, likely a standard central core leading to a standard numerical aperture.
Japanese Publication No. 2001-235648, which is hereby incorporated by reference in its entirety, is related to the compensation of the deleterious effect of the tail/skirt at the core cladding interface inherent to VAD process. The publication, however, fails to disclose a central core maximum refractive index and is, therefore, likely a standard central core leading to a standard numerical aperture.
R. E. Freund, “High-Speed Transmission in Multimode Optical fibers,” JLT, Vol. 28, No. 4, February 2010, which is hereby incorporated by reference in its entirety, discloses a double-alpha profile. However, this profile was set up to simulate manufacturing defects in the optical fiber. The continuity of the first derivative of this profile is not ensured and leads to poor DMD performances.
Graded-index multimode optical fibers are typically dedicated to high-speed data communications. They efficiently utilize low-cost high-speed sources based on VCSEL technology. Because these sources are divergent, multimode optical fibers are typically designed to provide a large numerical aperture—larger numerical apertures are generally better. Typical 50-micron-core multimode optical fibers dedicated to high-speed transmissions exhibit numerical apertures around 0.200.
Emerging optical-fiber applications like supercomputer or consumer electronic devices require additional flexibility that can be provided by an even larger numerical aperture. That said, conventionally a large numerical aperture generally leads to modal bandwidth degradation. Therefore, a need exists for a multimode optical fiber with high modal bandwidth and large numerical aperture.
Moreover, depending on applications, optical fibers with larger core size than a standard 50-micron core (e.g., an 80-micron core) may be used. Therefore, a need also exists for a multimode optical fiber having a large numerical aperture with high modal bandwidth regardless of core size, particular when the core size is increased beyond the standard 50-micron core.